1. Field of the Invention
The present invention relates to metal-oxide semiconductor (MOS) transistors, and more particularly, to an apparatus and method for adjusting the threshold voltage of MOS transistors.
2. Description of the Related Art
A metal-oxide semiconductor field-effect transistor (MOSFET) utilizes a thin dielectric barrier to isolate the gate and channel. A control voltage applied to the gate terminal induces an electric field across the dielectric barrier and modulates the free-carrier concentration in the channel region. MOS transistors are classified as either p-channel or n-channel devices, depending on the conductivity type of the channel region.
MOS transistors are also classified according to their mode of operation. In a depletion-mode MOS transistor, a conducting channel exists under the gate with no applied gate voltage. The applied gate voltage controls the current flow between the source and the drain by "depleting" or narrowing, a part of the channel.
In an enhancement-mode MOS transistor, no conductive channel exists between the source and the drain at zero applied gate voltage. As a gate-source bias of proper polarity is applied and increased beyond a threshold voltage V.sub.T, a localized inversion layer is formed directly below the gate. This layer serves as a conducting channel between the source and the drain. If the gate-source bias is increased further, the resistivity of the induced channel is reduced, and the current conduction between the source and the drain is enhanced.
In an n-channel enhancement-mode MOS transistor, the gate-source voltage V.sub.GS must be positive in order to induce a channel. Because no current can flow until the channel is formed, current will flow only when v.sub.GS exceeds a positive V.sub.T. In a p-channel enhancement-mode MOS transistor, the gate-source voltage v.sub.GS must be negative in order to induce a channel. Current will flow only when V.sub.GS falls below a negative V.sub.T, or, in other words, when V.sub.SG exceeds a positive V.sub.T.
The value of the threshold voltage V.sub.T of a MOS transistor is determined in part by the fabrication process specifications, i.e., the channel length, channel width, doping, etc. Thus, V.sub.T can be set to a desired level during fabrication.
In an integrated circuit that may contain many thousands of MOS transistors, various circuitry is employed to vary the gate-source voltage V.sub.GS of each transistor in order to switch the transistor on and off. Generally, V.sub.GS is increased above V.sub.T to switch the transistor on and decreased below V.sub.T to switch the transistor off. The circuitry used to vary the v.sub.GS of the individual transistors is configured to operate with predetermined supply voltages that are supplied to the integrated circuit.
Although conventional MOS transistors have functioned adequately in thousands of integrated circuit applications, there are currently at least two problems associated with threshold voltage V.sub.T selection which have contributed to the inefficiency of MOS transistors.
The first problem associated with V.sub.T selection relates to the predetermined supply voltages. Due to varying loads placed on voltage supplies and inconsistencies in various voltage supplies, it is not uncommon for a predetermined supply voltage that is supposed to be 3.3 Volts to be as low as 2 Volts or as high as 5 Volts. Thus, the V.sub.T of the transistors must be chosen so that the transistors will be operable with a voltage supply falling in the range of 2 to 5 Volts.
It is difficult, however, to choose the V.sub.T such that the transistor will operate efficiently in such a wide supply voltage range. For example, if V.sub.T of the transistors is chosen low for optimal performance when the supply voltage is 2 Volts, then a supply voltage of 5 Volts will cause v.sub.GS to increase too far above V.sub.T. Because the chosen V.sub.T may be too low for operation with a 5 Volt supply, V.sub.GS may be close or even equal to V.sub.T when the transistor is supposed to be switched off. Thus, if V.sub.T is too low, then the transistor might be slightly on resulting in current leakage when it is supposed to be in the off state.
On the other hand, if the V.sub.T of the transistors is chosen high for optimal performance (i.e., low leakage in the off state) when the supply voltage is 5 Volts, then a supply voltage of only 2 Volts will decrease the amount of "headroom" available in the operating range of the transistor. Specifically, the term "headroom" as used herein is intended to refer to the difference between the supply voltage and V.sub.T. If the chosen V.sub.T is too high, then the headroom is decreased which is undesirable because there is less of a guarantee that v.sub.GS will go far enough above V.sub.T to fully switch the transistor on. Indeed, it is possible that V.sub.GS may not even reach V.sub.T in which case the transistor would not be capable of switching on.
The second problem associated with threshold voltage selection relates to the fabrication process of MOS transistors. The channel length, channel width, gate oxide thickness, doping, etc., all play a part in determining V.sub.T. Although modern fabrication techniques permit V.sub.T to be defined fairly accurately, inconsistencies in the fabrication process nevertheless cause V.sub.T variations among the individual transistors.
Even if the supply voltage does not vary from its specified value, if V.sub.T cannot be accurately set, the same problems of current leakage or decreased headroom can occur. In other words, if V.sub.T comes out at the low end of the specifications, then current leakage may be a problem. On the other hand, if V.sub.T comes out at the high end of the specifications, then the headroom is decreased.
Conventional solutions to the inability to accurately control V.sub.T include choosing an extra large channel length L so that when V.sub.T comes out at the low end of the specifications, perhaps due to doping variations, current leakage remains within the specifications. However, this solution suffers from the disadvantage that, when an extra large channel length is used, the average drive current must be kept low in order to control current leakage. It is more desirable to have a higher average drive current.
It is becoming increasingly more desirable to use lower supply voltages with MOS transistors in order to conserve power. As supply voltages decrease, accurate control over the exact value of V.sub.T becomes more important. Accurate control over the value of V.sub.T is important because less headroom is available. Furthermore, if V.sub.T can be accurately controlled, then the transistor can be fabricated with process specifications that permit it to tolerate a higher average drive current.
Therefore, an apparatus and/or method is needed that will solve the problems associated with MOS transistor threshold voltage V.sub.T selection.